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Neocatastrophism and the Milky Way Astrobiological Landscape

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Neocatastrophism and the Milky Way Astrobiological Landscape Serb. Astron. J. } 176 (2008), 71 - 79 UDC 52{37 DOI: 10.2298/SAJ0876071V Preliminary report NEOCATASTROPHISM AND THE MILKY WAY ASTROBIOLOGICAL LANDSCAPE B. Vukoti¶cand M. M. Cirkovi¶c¶ Astronomical Observatory Belgrade, Volgina 7, 11160 Belgrade, Serbia E{mail: [email protected] (Received: February 13, 2008; Accepted: April 1, 2008) SUMMARY: The number and distribution of habitable planets in the Milky Way is one of the foremost problems of contemporary astrobiological research. We investigate the e®ects of applying general neocatastrophic paradigm to the evolution of the Galactic Habitable Zone. In this paper, we investigate the limits of simple, 1- dimensional astrobiological models, and consider the role of regulation mechanisms in shapening the "astrobiological landscape". We show that the transition from predominantly gradualist to predominantly (neo)catastrophist history of our Galaxy leads to the build-up of large-scale correlations between habitable sites, o®ering possible keys to such important problems as Carter's "anthropic" argument and Fermi's paradox. In addition, we consider the possibilities for extending the present class of models into spatially realistic 3-dimensional case via probabilistic cellular automata. Key words. Astrobiology { methods: numerical { Galaxy: evolution { extraterres- trial intelligence How many kingdoms know us not! Cirkovi¶c2004a,¶ Blair et al. 2008), as well as occa- Blaise Pascal, Thoughts, 207 sional criticism (Prantzos 2007). On the balance, it seems that it has o®ered a useful framework for for- mulation of some of the most general, far-reaching ideas about the emergence of life (biogenesis) and 1. INTRODUCTION intelligence (noogenesis) on the Galactic scale. In the present paper, we study the class of simplest, 1- The concept of the Galactic Habitable Zone dimensional astrobiological models of GHZ, examine (henceforth GHZ) emerged as one of the crucial their limits of application and discuss a particular hypothesis about the general astrobiological evolu- terms of the contemporary astrobiological research tion in the Milky Way, namely the neocatastrophic (often dubbed, with much justi¯cation, "the astro- view. biological revolution"). Starting with the pioneer- The present approach seeks to join the older ing paper by Gonzalez et al. (2001), the concept tradition of more specialized SETI-oriented models, experienced signi¯cant elaboration and generaliza- as exempli¯ed by the studies of Newman and Sagan tion (e.g. Lineweaver et al. 2004, Gonzalez 2005, (1981), Fogg (1987) or Landis (1998) with the recent 71 B. VUKOTIC¶ and M. M. CIRKOVI¶ C¶ astrobiological developments, notably GHZ elabora- timescale is much larger than the astrophysical one, tions and the re-emergence of (neo)catastrophic view thus making the evolution of life and intelligence else- of the large-scale biospheric evolution. Some results where correspondingly improbable. of this kind of modeling have been presented by 3. The argument from biological contingency Vukoti¶cand Cirkovi¶c(2007,¶ henceforth Paper I) and (Simpson 1964, Mayr 1993): the part of biological Vukoti¶c(2008, henceforth Paper II). The application morphospace containing intelligent beings capable of of these results to resolving the famous Carter's ar- SETI-like communication is so minuscule in volume, gument against extraterrestrial intelligence has been that even if all habitable planets in the Galaxy are ¶ indeed inhabited by life of some kind, SETI will still presented in Cirkovi¶cet al. (2008). Finally, the res- be condemned to failure. olution of Fermi's paradox through the phase transi- The "Big Three" arguments against SETI tion scenario encapsulated by these models is pre- have played a large role not only in academic discus- sented in Cirkovi¶cand¶ Vukoti¶c(2008). Here, we sions about these topics, but also in its wider cultural would like to elaborate upon the applicability con- and societal perspectives (i.e. Mayr 1993), includ- ditions of such stochastic models, properties of the ing the US government budget cuts for SETI. They relevant regulation mechanisms, as well as their em- have been influential in formulating the controver- bedding into a more general and realistic 2-D and sial, but very popular in the astrobiological circles, 3-D astrobiological modeling framework. "rare Earth" hypothesis (Ward and Brownlee 2000, Conway Morris 2003). As has been argued elsewhere ¶ ¶ 2. MOTIVATION FOR 1-D (Cirkovi¶c2004ab, Cirkovi¶cet al. 2008), these ar- guments are inconclusive, but very instructive and NEOCATASTROPHIC MODELS should present a strong motivation for the develop- ment of theoretical SETI. We know very little about the details of what The present approach can be regarded as a could be dubbed "astrobiological dynamics", namely way of direct simulation testing of the famous Drake the laws governing biogenesis and noogenesis, the de- equation, developed by Frank Drake for the ¯rst pendence of the speed of evolution and size of the SETI symposium in 1961. In a sense, it is one of the morphological space on the environmental conditions very rare theoretical SETI results. Although there in general case, etc. Even in such situations, as for is no canonical form of the Drake equation, and the instance the history of many-body physics shows us, expression quoted by various authors is often depen- one can learn a great deal about the large-scale, col- dent on the desired result of the analysis, we use the lective behavior of the system through Monte Carlo following form (e.g. Shklovskii and Sagan 1966, Wal- simulations, which neglects local details and gives ters et al. 1980): information only on the most general trends. Along the line of the same idea, we have constructed a se- N = R¤fgfpneflfifcL; (1) ries of 1-dimensional models, where we are only in- terested in the number of inhabited planets in any while keeping in mind that other equivalent forms given epoch, while the local biological timescales are exist. In this expression, the symbols have the fol- chosen at random from a su±ciently broad tempo- lowing meanings: ral interval. Speci¯cally, we have been motivated by the idea of "simulating" the important Carter's N = the number of Galactic civilizations with anthropic argument against extraterrestrial intelli- which communication is possible. R¤ = mean rate of star formation in the gence (Carter 1983). Carter's argument is one of the Galaxy, three "classical" (in the sense of being discussed in fg = fraction of stars suitable for supporting most of the SETI-era starting from early 1960s) anti- life, SETI arguments. These are (in somewhat simpli¯ed fp = fraction of stars with planetary systems, terms): ne = number of planets per planetary system 1. Fermi's paradox (Brin 1983, Jones 1985, with conditions ecologically suitable for the origin Webb 2003): Any early spacefaring civilization and evolution of life, should have colonized the entire Galaxy, includ- fl = fraction of suitable planets where life orig- ing the Solar System and Earth by this late time inates and evolves into more complex forms, in Galactic history. This is not what we observe fi = fraction of planets bearing life with intel- ("Great Silence"). ligence, 2. Carter's anthropic argument (Carter 1983, fc = fraction of planets with intelligence that 1993, Livio 1999): If biological and astrophysi- develops a technological phase during which there is the capability for an interest in interstellar commu- cal timescales determining the evolution of life and nication, intelligence are independent, we have a posteri- L = mean lifetime of a technological civiliza- ori Bayesian reasons to believe that the biological tion. 72 NEOCATASTROPHISM AND THE MILKY WAY ASTROBIOLOGICAL LANDSCAPE Drake's equation is usually used as a rule- Annis (1999a) noticed that gamma-ray bursts of-thumb estimate in order to assess feasibility of (GRBs) seem to satisfy all three desiderata. These SETI projects covering ¯xed number of targets. It strongest explosions in the universe have several ways has many weaknesses, repeatedly elaborated by both of adversely influencing biospheres over a large range contact-pessimists and optimists (e.g. Lem 1977, of distances within their host galaxies (Thorsett Brin 1983, Gould 1987, Fogg 1988, Cirkovi¶c2004b).¶ 1995, Scalo and Wheeler 2002, Melott et al. 2004). However, in order to assess validity of its estimates, The decrease of their frequency in time, roughly fol- we need to compare them with a more detailed nu- lowing an exponential trend, recoverable from cosmo- merical model and, since the only value we are in- logical observations, makes the astrobiological clocks terested in is the total number of targets N, the 1-D in GHZ running longer and longer without reset. models like the one we propose are satisfactory can- This, in turn, virtually guarantees that at some point didates. Various f-parameters in Drake's equation there will be a sudden "phase transition" between are, essentially, ¯lters for habitability; a large part mostly dead Galaxy and the Galaxy teeming with of them can be thought of as regulation mecha- life (and, presumably, viable SETI targets). It is this nisms influencing the local evolution on habitable aspect of Annis' conjecture rather than the speci¯c planets in GHZ. Astronomical factors in Eq. (1) are regulation mechanism he advocated that renders his nowadays reasonably well-understood due to the im- theoretical approach genuinely novel. mense progress in astrobiological research in recent Obviously, the increase in level of technologi- years; notably, this applies to R, fg, fp and, in sig- cal sophistication which, in accordance with our as- ni¯cantly lesser degree, ne. As a result of such de- sumptions, is just the extension of the increase in velopment, main uncertainties left lie with biological biospherical complexity will|if unimpeded for quite and "sociological" factors, namely fl, fi, fc and { in a brief period beyond the present human level|lead particular { L. to the security of intelligent community (and, indeed, most of the domicile biosphere) against the GRB threat.
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